The present invention relates to a shift control system for an automatic transmission, and more particularly to a hydraulic system for alleviating shocks taking place during a shift involving activation of dual pistons of a servomotor of a friction device.
JP 62-62047 discloses a shift control system for an automatic transmission. This automatic transmission is shiftable between any two of four forward speed ratios, and it includes a band brake with a servomotor having dual pistons. This band brake is applied during operation with the second speed ratio and during operation with the fourth speed ratio. The servomotor of this band brake, therefore, has two apply chambers which the dual pistons are exposed to, and a release chamber. One of the two apply chambers is adapted to be supplied with hydraulic pressure during a shift to the second speed ratio, and thus it is hereinafter called as a second speed apply chamber. The other apply chamber, on the other hand, is adapted to be supplied with hydraulic pressure during a shift to the fourth speed ratio, and thus it is hereinafter called a four speed apply chamber. The band brake is applied during operation with each of the second and fourth speed ratios. The release chamber is adapted to be supplied with hydraulic pressure during a shift to the third speed ratio, causing the band brake to be released. There is provided an accumulator in communication with a hydraulic line leading from a 1-2 shift valve to the second speed apply chamber to induce a gradual rise in hydraulic pressure building up in the second speed apply chamber during a 1-2 upshift. Similarly, another accumulator is provided in communication with a hydraulic line leading from a 3-4shift valve to the fourth speed apply chamber to induce a gradual rise in hydraulic pressure building up in the fourth speed apply chamber during a 3-4 upshift.
However, the above-mentioned known shift control system poses a problem that the capacity of the band brake increases excessively although momentarily during a 1-4 upshift. The 1-4 upshift is initiated by simultaneous supply of hydraulic fluid to the second speed apply chamber, third speed release chamber, and the fourth speed apply chamber. The hydraulic pressure build-up in the third speed release chamber, however, is slow, as compared to the hydraulic pressure build-up in each of the second and fourth speed apply chambers. Thus, the torque capacity of the band brake increases momentarily to an excessively high value that is determined by the hydraulic pressures acting in the second and fourth speed apply chambers. This induces a great shock. The reason why the hydraulic pressure build-up in the third speed release chamber is slow is that the hydraulic fluid supplied to the third speed release chamber is also supplied to a high clutch and thus there is a delay until the hydraulic pressure in the third speed release chamber increases sufficiently. This excessive increase in the torque capacity taking place momentarily during the 1-4 upshift may be avoided if the setting pressure of the accumulator provided in communication with the hydraulic line leading to the fourth speed apply chamber is made sufficiently low. However, this approach to solve the above-mentioned problem impairs quality of a 3-4 shift because the hydraulic pressure build-up in the fourth speed apply chamber becomes slow during the 3-4 shift owing to the accumulator, resulting in an increase in the length of time for the 3-4 upshift, inducing engine racing.
An object of the present invention is to improve a hydraulic control system of the above-mentioned type such that the above-mentioned problem is solved without any major change in the existing component parts of the system.